Preheating of aluminum scrap prior to being fed into a gas or oil fired reverberatory furnace for processing is known to drive off moisture carried on the scrap. If the water is not driven off, explosive vaporization can occur when the scrap is introduced into the furnace. Such preheating also reduces the cost of processing of the metal by raising the temperature of the metal before its being introduced into the furnace.
In previous proposals, cold scrap was placed on a sill surrounding the well of the reverberatory furnace which carried the molten metal, before being pushed into the well to preheat the scrap and drive off the water. In some instances, a carriage was pushed into a position near the mouth of the furnace at the level of the sill to preheat the scrap on the carriage before being pushed onto the sill for more heating before introduction into the well.
In another proposal, scrap iron is pushed by pushers from step to step in a preheat area of a furnace before discharge into the furnace hearth (see Canadian Letters Pat. No. 645,586).
In more recent proposals, preheating of scrap aluminum has been accomplished by applying heat generated by special burners or waste heat carried by the flue gases discharged up the stack. In U.S. Pat. No. 3,669,436, scrap metal carried in a container is preheated at a preheating station by waste gases passed through the bottom of the container up through the scrap. Thereafter, the contained scrap is discharged into the furnace. Of course, it will be appreciated that such a proposal is costly (particularly for the amount of material that can be preheated) and complex. U.S. Pat. No. 3,424,445 discloses another approach where containers carrying scrap metal are moved on trucks in a gas tunnel carrying waste gases.
In more recent proposals, a rotary kiln has been used because of, its compact size, adaptability to high capacity processing (unlike the proposals previously discussed) and,capability of achieving uniform heating of the aluminum scrap. However, rotary kiln applications are limited to smaller sized scrap (for example, chopped, crushed or shredded). Therefore the scrap (unless of small size) before preheating in the rotary kiln, must be shredded, comminuted or chopped into smaller pieces. If the aluminum scrap for example is merely compressed, water may be trapped and/or the metal scrap will not be preheated thoroughly. Furthermore, where the scrap must first be shredded, chopped or otherwise comminuted, the preliminary processing requires the expenditure of additional energy and which processing results in about 1% -2% loss of metal. In preheating the small sized scrap metal in the rotary kiln, the kiln lifts and tumbles the scrap as flue gases are passed over and around the metal pieces.
In another proposal, small pieces of scrap metal (shredded cans, borings, turnings, extrusions, plates and small castings) are preheated also by the use of waste heat. In this proposal the preheater comprises a downwardly inclined chamber having an inclined bed of overlying spaced plates on the bottom thereof permitting flue gases to be passed into the hopper through the spaced plates to engage the pieces of scrap metal passing down through the inclined chamber to preheat the scrap metal. The preheated metal is discharged at a controlled flow rate (controlled by vibratory discharge) through an inclined discharge chute. The additional drawbacks with this process (over and above those previously described) include the jamming of the scrap metal in the chamber and the compaction of the small pieces of metal in the chamber as they pass through the chamber so that the flue gases do not touch all the scrap metal to fully preheat it.
Thus it is apparent that not one apparatus has been proposed which preheats large pieces of aluminum scrap metal without first requiring the cutting up of the large pieces or being cost prohibitive before processing in the reverberatory furnace.
Where compacted bundles of large pieces of aluminum scrap metal are brought in for processing into aluminum billets, they must be dealt with in such a way that no water is inadvertently introduced to the reverberatory furnace. To ensure water is eliminated, preheating is desireable. However, the preheating must be done at minimum cost with maximum efficiency.
It is therefore an object of this invention to provide an improved preheater especially suitable for preheating large irregular pieces of aluminum scrap metal.
It is a further object of this invention to provide such improved preheater which is efficient and entirely reliable yet of minimal cost and of minimum complexity.
Further and other objects of the invention will be realized by those skilled in the art from the following summary of the invention and detailed description of embodiments thereof.